This invention relates to a ceramic/metallic composite and method for producing the same, based on the self-propagating high-temperature synthesis (SHS) and such compact which comprises superabrasive particles of diamond or high pressure polymorphism of boron nitride, or cBN or wBN, dispersed in a matrix of metallic and/or ceramic matrix.
Ceramic/metallic composites as a novel material which show a combined nature of ceramic and metal are useful for application to construction materials or the like. For manufacture, a technique called SHS is known, which is based on the process which occurs with a certain material system: a combustion, once initiated by igniting at a spot, sustains itself and propagates throughout the rest of the material, due to an intense production of heat which spreads and causes a sufficient temperature rise. This technique advantageously facilitates the preparation of a substance of melting point over 2000xc2x0 C., and useful for the production of functional materials such as carbide, nitride, boride, silicide or oxide of the fourth or fifth group metals of the periodic table, including Ti, Zr, Ta, Si, as well as intermetallic compounds. The technique is fully described in xe2x80x9cThe chemistry of SHSxe2x80x9d, published by T.I.C. (1992).
It is known that the synthesis process can be initiated by:
(1) putting into contact with an electroresistive heater or like a pellet, or formed compact body, of mixed powder so composed as to be capable of SHS (direct ignition) and igniting at an end;
(2) providing, in contact with the pellet at an end, another mixed powder of SHS capable composition; the latter is ignited and the resulting intense heat is used to ignite, in turn, the pellet (two-step ignition); and
(3) heating with a heat source (electroresistive element, for example), which is provided around, so as to ignite the formed compact body (indirect ignition).
Of the three above, the techniques (2) and (3) are commonly employed for the purpose of facilitating ignition to the pellet body when the principal heat source is insufficient for the self-propagation of the process: it uses as a secondary heat source a premixed powder which is provided around the compact body as described above.
Thick or massive composites can be obtained by an SHS-based sintering technique when the SHS products either melt or soften. However it is hardly the case with titanium carbide composite, for example, which is obtained as a porous product, when prepared with a starting mixed powder of metal and carbon. Probable reasons are: (1) the carbide product has a melting point too high, and (2) a solid state reaction can take place between Ti and C at rather a low temperature, and produces TiC which forms a firm network. Here densification may be achieved to a degree by compression, but it is impossible to obtain a product substantial free of pores. This is also the case with the synthesis of other high melting materials.
An SHS process, which can produce high temperatures over a short period of time almost adiabatically, is employed for the formation and sintering, simultaneous or subsequent, of high melting materials and, if tentatively, for the preparation of compact of various materials. For the materials, these techniques are available: static compression with a mechanical press, instantaneous compression by explosive detonation, isostatic compression with a HIP system, pseudo-HIP process whereby the formed compact is squeezed from around with a mechanical press in a die by means of molding sand.
On the other hand, composites of diamond or high pressure polymorphism of boron nitride (cBN or wBN), either interjoined or dispersed and held in the matrix, are widely employed in industries. The superabrasive substances are metastable under normal pressure and more so at high temperatures such as used in the sintering process: they both can transform rapidly to the low pressure polymorphism, that is graphite or hexagonal boron nitride, so an ultrahigh pressure is needed in order to prevent such unfavorable transition processes by establishing a condition where those superabrasive substances are thermodynamically favored phases, and pressures of several (more than four, commonly) giga-pascals are exerted in sintering processes. Thus the volume available poses limitation to the largest product dimensions achievable, which is currently around three inches (76.2 mm) in diameter.
Some less firm-structured wear-resistant materials wherein super abrasive particles are not necessarily interjoined, are tentatively prepared by means of either HIP or hot pressing, instead of the ultrahigh pressure technique. These processes are advantageous in that larger wear-resistant products can be achieved as essentially free of equipment related limitation, while in the manufacture of heat resistant composite products a maintained temperature of 1000xc2x0 C. or more is needed for several minutes for the densification of the matrix, due to the nature of the process and implementation. This inevitably causes transition, partly or essentially, of the superabrasive to the low pressure phase, and resulting deterioration in particle properties and retention to the matrix make it difficult for these wear-resistant materials to be substituted for those ultrahigh pressure compacts.
SHS processes cause often a temperature rise to more than 2000xc2x0 C., although heat evolves usually over a short period of several seconds. No techniques have been available, as far as the Inventors are aware, which could apply such almost instantaneous heating to apply in the manufacture of compacts, of composite or not, which comprise superabrasive particles.
Therefore, one of the principal objects of the present invention is to provide an effectively densified compact body of titanium carbide and other ceramics which are difficult, as described above, to be produced as a dense compact by SHS, and in particular, such construction material which consists of a ceramic skeletal structure, densified with metallic phase which is filled in the internal gaps by penetrating with fused Tixe2x80x94Al alloy (or intermetallic compound). Another object of the invention is to provide an adequate method for the preparation of such compacts.
Another object is to provide a novel superabrasive containing compact which is essentially free of heretofore inevitable limitation in the product size and deterioration in abrasive strength. A further object is to provide an effective method of producing such compacts.
The first aspect of the invention is a sintered composite body which comprises an integrated body or several refractory pieces or ceramic particles and metallic material filling the gaps within and among them, with the former being selected from carbides, borides, nitrides, and silicides of Ti, Zr, Ta, Nb, Si, Cr, W and Mo and joined three-dimensionally, and the latter comprising one selected from binary alloys and intermetallic compounds of the systems of Tixe2x80x94Al, Tixe2x80x94Ni and Nixe2x80x94Al. The composite can be prepared basically by: admixing powder of Al and/or Ni metal to a metal/non-metal powder mixture so formulated as to be capable of producing such refractory products by SHS, mixing and molding said powder to form a pellet, placing the latter in a die, initiating an SHS process within the pellet to cause fusing, at least partly, and softening of said both metals by the heat produced thereby and forming the skeletal structure of refractory compound, while filling the skeleton gaps with the flowing metal.
The heat volume to be produced depends on the starting chemical system, so while it may be essentially sufficient for sustaining the process of skeletal formation and causing the metallic material to flow and fill the gaps with one composition, it may be not with another. Such problem can be eliminated by the so-called chemical oven technique, whereby another chemical system or powder composition is provided for an SHS process and supplementary heat is supplied from around the formed pellet.
It is now possible to effectively achieve superabrasive particles of good workability in various densified matrix of composite, by adding the particles to the formed pellet and employing an SHS process to serve as a heat source for causing the matrix to flow, under a pressure-temperature condition where diamond is metastable. The heat source chemical system may be comprised either as matrix components or in adjacency of a matrix composition which is essentially free of heat production. In the both cases, the matrix comprises certain components which can melt or soften under the high temperature conditions provided by the SHS process. The fused or softened materials are compressed to make a composite body of densified structure. The compression is started slightly after the completion of the SHS process. Superabrasive particles may be comprised, as desired, partly on or in the surface or uniformly in the whole volume of the resulting composite, for example.
Single or multilayered coating of metallic and/or ceramic material may be deposited on the superabrasive particles for performing an improved retention to the matrix and, thus, improved grinding performance.
In the process of invention, graphitization of diamond superabrasive and resulting deterioration in either particle properties or retention to the matrix is minimal and negligible, since the high temperature reaction is accomplished to terminate within a very short period. Further, in spite of frequent observations, with conventional manufacturing techniques of grinding tools with diamond particles, that pitting corrosion occurs on diamond particles when placed and heated in contact with titanium or other transition metal component in the matrix, due to the reaction of carbon and metal, such corrosion of diamond particles and resulting deterioration in particle strength is minimized and there are essentially no related problems any more, since very little mass of carbon moves from the diamond to the metal within such a very short time the carbide forming process lasts.
Moreover, in the invention, graphitization of diamond particles proceeds only insignificantly even if arranged and heated in contact with such harmful graphitizer metal as Fe, Ni or Co as a support material or a filler or other component of the matrix, since the reaction process lasts only for a very short time, as described above. This is probably due to the short heating time for the diamond, as well as the formation on the diamond of TiC film, which serves as a barrier to the diffusion of carbon and iron-group metal atoms.
The maximum attainable temperature in the SHS materials during the process can be estimated essentially by the adiabatic combustion temperature for the formation of the compound. In case the estimated temperature by far exceeds 2000xc2x0 C., the SHS material should be diluted with components neutral to the process, in order to prevent the transition of the superabrasive to the corresponding low pressure polymorphism.